Datacite sizes

Addresses https://trello.com/c/54t4olTA/214-feat-add-file-sizes-to-datacite-and-dataone-metadata

I also refactored the crosswalk to clean up the code. I think it still works; I validated it against 1) embargoed data file, 2) public data file, 3) public data package.
Since this doesn't clear existing metadata, won't we get duplicate values?